Incomplete Dominance, Codominance, Multiple Alleles,

and Sex-Linked Genes

Incomplete dominance

• This is when neither allele is dominant.• Both alleles are expressed and contribute

equally to the phenotype.

• A heterozygous genotype has an intermediate phenotype as there is partial influence from both alleles.

• Example: Red + White = Pink flower flower flower

Incomplete dominance

• Snapdragons can be – red (alleles = RR) – white (alleles = rr) – pink (alleles = Rr)

• If you cross a two pink flowers, is it possible to produce a white flower?

¼ Red

¼ White

½ Pink

Yes!

Codominance

• Both alleles are dominant.

• They are independent, so there is no 'blending' as in the snapdragons, instead the phenotype is a result of the full expression of both alleles.

• Example: black + White = B&W spotted dog dog dog

Codominance

• Dogs can be• Black (alleles = BB)• White (alleles = bb)• Spotted (alleles = Bb)• Is it possible for a black dog to have one

parent that is white?No!

Multiple alleles (blood typing)

• In the case of the ABO blood grouping, there are 3 alleles for one gene

• They are written a little differently:

• i : this allele is recessive• IA : this allele is co-dominant• IB : this allele is co-dominant

Multiple alleles (blood typing)

• Blood group (phenotype):

• A ……………………………..

• B ……………………………..

• AB ……………………………

• O ……………………………..

Possible genotype:

IA IA or IAi

IBIB or IBi

IAIB

ii

Multiple alleles (blood typing)

• A father with homozygous type A blood has a child with a mother that has heterozygous type B blood.

• Is it possible for them to have a child with type O blood?

Sex determination

• Gender is determined by sex chromosomes in many animals.

• The XY System: – Female are XX, males are XY

• Y makes the guy!!!

• What are the odds that a couple have a boy?

50/50

Sex-linked genes

• Some genes are linked to the sex chromosomes and are inherited with them.

• Hemophilia is carried on the X chromosome.

• The normal allele is dominant (H)• The allele for hemophilia is recessive (h)

Sex-linked genes

• XHXH = normal female• XHXh = carrier female• XhXh = female w/hemophilia• XHY = normal male• XhY = male w/hemophilia

A carrier does not have the disease, but can pass it on to their offspring.

Sex-linked genes

• If a female carrier has offspring with a normal male, what is the chances that their offspring will have hemophilia?

25%

Sex-linked genes

• In cats the allele for coat color is carried on the X chromosome.

• The alleles are black and orange but they are codominant.

• XBXB = black female• XBY = black male• XbXb = orange female• XbY = orange male• XBXb = tortoiseshell female

Sex-linked genes

• Can an orange male and a black female produce a tortoiseshell male?

No!

Sex-linked genes

What is the genotype of the father?

Sex-linked genes

• Mother can provide XB or XO

• Father can provide Y plus 1 other allele.• Since 1 offspring is XO XO, the father must

provide 1 of these allele.• Therefore, the father's genotype is XOY